Electroplating a ductile zinc-nickel alloy onto strip steel



E. J. ROEHL 3,420,754

ELECTROPLATING A DUCTILE ZINC-NICKEL ALLOY ONTO STRIP STEEL Jan. 7, 1969I Of5 Sheet Filed March 12. 1965 IZOOO IOOOO EFFECT OE DEPOSITCOMPOSITION ON INTERNAL STRESS OF CO-DEPOSITED Zn-Ni ELECTROPLATEPERCENT NICKEL IN DEPOSIT F lg.

7 INVENTOR- EDWARD J. 'ROEHL ATTORNEYS United States Patent 3,420,754ELECTROPLATING A DUCTILE ZINC-NICKEL ALLOY ONTO STRIP STEEL Edward .I.Roehl, Warren, Ohio, assignor to Pittsburgh Steel Company, Pittsburgh,Pa., a corporation of Pennsylvania Filed Mar. 12, 1965, Ser. No. 439,268

U.S. Cl. 204-28 3 Claims Int. Cl. C23b 5/04 ABSTRACT OF THE DISCLOSUREThe invention is concerned with the electrodeposition on steel strip ofa Zinc-nickel alloy plating containing nickel in a predetermined rangeof 6.5 to 9.5%, for the purpose of providing a coating having anexpansive internal stress of about 500-1400 p.s.i., whereby a highductility and low internal stress enables the plated strip to besubjected to cutting, bending, stamping or forming operations withoutcracking or breaking of the plating. At the same time, superiorcorrosion-resistant properties for the plating are obtained.

This invention relates generally to the electrodeposition of alloys fromacid plating baths, but has reference more particularly to theelectrodeposition of alloys or co-deposits of zinc and nickel, thecomposition of which is controlled within specific, fairly critical,limits or ranges for the purpose of obtaining new and desirable physicalproperties for specific, useful, commercial applications.

Zinc is one of the most widely used metallic coatings for steel surfacesto protect such surfaces from corrosion, the principal methods ofapplying such coatings being hot dipping and electroplating. The hotdipping method produces thick coatings of a thickness of 0.001 inch ormore, which are alloyed to the surfaces of the steel. These intersurfacealloys are brittle and not suitable for drawing and forming operations.Electroplating produces thinner coatings, of the order of a fewten-thousandths of an inch in thickness, with no alloy intersurfacelayer between the base metal coating, and hence can be formed and drawnsatisfactorily.

Zinc has been electroplated on steel surfaces from acid plating bathsfor many years for the purpose of providing protection of the steelsurface for commercial uses. Among commercial materials so protected iscontinuous steel strip which, after being plated, is fabricated intouseful articles of manufacture by cutting, stamping, drawing and formingoperations. In order to withstand the stresses of such operationswithout damage or injury to the zinc plating or interfering with itsadhesion to the steel strip or sheet and the corrosion protectionoffered by the zinc, it is essential that the electroplated zinc coatingpossesses minimum internal stress and maximum ductility. Heretofore,these conditions or requirements 'have been best met by eliminatingimpurities from the plating bath, so as to deposit zinc of the highestpurity.

It has also been proposed, as in U.S. Patent No. 2,419,231, to producean electrodeposited coating of an alloy high in zinc and low in nickel,by the addition of nickel salts to an acid zinc plating bath, whereby aplated coating is provided, the resistance of which to corrosion issuperior to that provided by pure zinc alone when 3,420,754 PatentedJan. 7, 1969 ice plated on steel surfaces. The patent is concernedprimarily with alloys consisting of about zinc and about 15% nickel,which alloys provide better corrosion resistance or protection than purezinc but which coatings themselves are not as ductile as pure zinc, andhence are not as suitable as pure zinc for forming and drawingoperations.

Efforts to use the invention of the aforesaid patent, especially for theplating of continuous steel strip, have met with almost insurmountablepractical difficulties, for two reasons. The preferred range of alloycompositions in the aforesaid patent for maximum corrosion protection is11-18% nickel, with the remainder zinc. In the first place, it isdifficult to secure good adhesion of the plated alloys of thiscomposition range, and this difiiculty is recognized in the patent whichrecommends the use of a pre-plate or strike of pure nickel on the steelprior to alloy plating to insure good adhesion. Such pre-plates orstrikes are expensive to apply commercially because two plating ssytemsare required, thus adding materially to the cost of the product, so thatthis additional plating operation is to be avoided whenever possible. Inthe second place, a far more serious difficulty has been encountered dueto the low ductility and high internal stress inherent in electroplatedalloys of zinc and nickel within said preferred range of 11 to 18%nickel. When continuous steel strip, plated in accordance with theteachings of the aforesaid patent, is subjected to cutting, bending,stamping or forming operations in the manufacture of articles for whichpurpose it is made, the plating cracks or breaks or becomes separatedfrom the steel base, because it is brittle and has relatively highinternal stress. This defect destroys the corrosion-resistant propertiesof the electroplated alloy, and consequently the commercial value of theplated product.

I have found that by carefully controlling the plating bath compositionand the operating conditions under which the high Zinc-low nickel alloyis plated, I can produce alloys within a narrow and critical compositionrange outside the preferred range disclosed in the aforesaid patent, andwhich have a surprisingly lower internal stress and higher ductility, aswell as satisfactory adhesion to the base metal, than does pure zincitself, and, at the same time, possess corrosion-resisting propertiessuperior to pure zinc characteristic of high zinc-low nickel alloys.

These improvements are best illustrated in FIG. 1 of the accompanyingdrawings, forming a part of this specification, which shows thatelectrodeposited alloys within a narrow range of 6.59.5% nickel, withthe remainder zinc, and preferably within a range of 79% nickel, withthe remainder zinc, have an expansive internal stress of about 500-1400p.s.i., as compared with an expansive internal stress of about 500p.s.i. for pure electroplated zinc, while electroplated nickel alloyscontaining over 9.5 nickel have a rapidly rising contractile internalstress, with corresponding loss of ductility, which accounts for thegreat difficulty encountered in working or forming products plated withthem, as hereinbefore referred to. FIG. 1 further indicates that apreferred alloy or composition within the selected range, and which hasa maximum expansive internal stress of about 1400 p.s.i., is oneconsisting of about 92% zinc and about 8% nickel.

Internal residual stress is stress present in a body that is free ofexternal forces or thermal gradients. The subject of internal stress, inconnection with electroplating,

3 is discussed on pp. 386-388 of the second edition (1962) of A. KennethGrahams Electroplating Engineering Handbook, to which reference may bemade.

The coated material made in accordance with my invention, having acoating of the composition described above, can be bent, stamped, drawn,formed, or otherwise Worked into finished articles or products withoutcracking or damaging the coating in any manner.

In FIG. 2 of the accompanying drawings, the results of salt spray testson flat specimens of coated strip steel, coated respectively with theaforesaid 92%-8% nickel alloy, pure electrolytic zinc, and hot-dippedzinc, are depicted graphically, and in FIG. 3 of the accompanyingdrawing, the results of the same salt spray tests, on similar specimens,after forming, are depicted graphically. In both cases, the 92% zinc-8%nickel alloy coating is seen to be from three to four times ascorrosionresistant as the electrolytic zinc and hot-dipped zinccoatings, and can therefore be used in thickness of only onethird toone-fourth those of pure zinc, for corresponding degrees of protection,thereby effecting considerable economies.

This discovery was most surprising and unexpected. The invention,therefore, simply stated, comprises plating or electrodepositing ahighly corrosive-resistant alloy of zinc and nickel, of minimum internalstress and maximum ductility, on steel surfaces, said alloy having acarefully controlled composition within narrow critical limits achievedby careful control of the plating bath composition and operatingconditions, thus producing a new and useful product at low cost.

In carrying out the invention, the important factor in plating bathcomposition and control is to maintain the ratio between theconcentration of zinc metal in solution and the concentration of nickelmetal in solution with respect to total metal concentration andoperating conditions so that the composition of the alloy platedtherefrom will be as close to 92% zinc and 8% nickel as possible, andpreferably within the limits of 93-91% zinc and 79% nickel, but neverless than 90.5% zinc or more than 9.5% nickel. I have found that thisobjective can best be accomplished by maintaining the zinc metalconcentration in the bath at between 8 and 10 ounces per gallon, withthe nickel metal concentration at between 4 and 5 ounces per gallon, andthe ratio of zinc metal concentration to nickel metal concentration asclose as 2:1 as possible.

The anionic composition of the plating bath is not critical, hence anywater-soluble salts of zinc and nickel commonly used in plating bathsand compatible therewith can be used alone or in any desirablecombination, although I prefer to use a chloride bath because of itshigher conductivity and therefore greater productivity. Other saltswhich may be used, however, include sulfates, sulfamates,

fiuoborates, and in some cases, acetates. I use small concentrations ofacetates or acetic acid, for example, as a buffer to promote ease of pHcontrol.

The following example illustrates a preferred embodiment of myinvention:

1 Not critical.

As anodes, I prefer to use separate zinc and nickel anodes, althoughalloys consisting of approximately 92% zinc and 8% nickel can be used ifdesired. In using separate anodes, I maintain the ratio of approximately92% of zinc surface to 8% of nickel surface and to manipulate this ratioas required to maintain the ratio of zinc metal concentration to nickelmetal concentration in the bath as near 2:1 as possible and thusmaintain the composition of the electrodeposited alloy within thedesired limits of 93-91% Zinc and 7-9% nickel, preferably 92% zinc-8%nickel.

Wetting agents which lower the surface tension of the bath may be addedthereto if desired to eliminate pitting, if encountered, and to improvethe uniformity of deposit appearance. The identity of the wetting agentschosen is not important, although they must, of course, be compatiblewith the bath and cause no deleterious effects. Some of the wettingagents used in commercial nickel plating have been found to besatisfactory, as for example, sulfated or sulfonated lauryl alcohol, oneof the commonest so used. Others which produce less foam may bepreferred, but the selection of such wetting agents most suitable forspecific conditions is well within the knowledge of those skilled in theart of electroplating, there being a wide variety available frompurveyors of nickel plating process.

Mild agitation of the bath, or relative movement between bath andcathode such as obtained by cathode movement or bath circulation isadvantageous in maintaining uniform bath composition and thereforeuniform deposit composition. The passage of continuous steel stripthrough the plating bath while it is being plated is entirely adequateand bath circulation through a pump and filter is desirable to maintainthe bath clean as well as in motion.

Although the invention has been described particularly with reference tothe electroplating of continuous steel strip, for which it is especiallyadapted, it is equally suitable for electroplating any steel surface forprotection against corrosion, as for example, sheet steel, steel pipes,conduits, tubing, wire, and any object that can be protected by zincelectroplating.

It is understood that slight changes may be made in the method and inthe alloys and compositions and products, as described, withoutdeparting from the spirit of the invention or the scope of the appendedclaims.

Having thus described my invention, I claim:

1. The method of plating steel strip with a zinc-nickel alloy whichcomprises: causing the strip to traverse an aqueous plating bath havinga pH of from 2.5 to 3.5, in which nickel chloride and zinc chloride havebeen dissolved in sufiicient amounts for each gallon of the bath to havea zinc content of from 8 to 10 ounces and a nickel content of from 4 to5 ounces; making said strip a cathode as it passes through said bath andmaintaining an electroplating current density of from to amperes persquare foot of cathode surface of the strip, whereby a zincnickel alloycoating is electrodeposited on the steel strip, said zinc-nickel alloyconsisting of from 6.5 to 9.5% nickel with the remainder zinc, saidcoating having an expansive internal stress of about 500-1400 psi. andwhich coating also has substantially greater corrosion resistance thanzinc.

2. The method, as defined in claim 1, in which said bath contains asurface tension lowering agent which is compatible with the bath.

3. The method, as defined in claim 1, in which the plating bath ismaintained at a temperature of from to F.

References Cited UNITED STATES PATENTS 1,928,053 9/1933 Freeman 75-1782,419,231 4/1947 Schantz 20443 XR 2,989,446 6/1961 Hammond et a1.20'4-43 XR 3,064,337 11/1962 Hammond et al. 29196.5

(Other references on following page) 5 FOREIGN PATENTS 940,302 5/ 1948France.

OTHER REFERENCES Schoch, Eugene et al., The Electrolytic Deposition of 5Nickel-Zinc Alloys," Journal of the American Chem. Soc., vol. 29, pp.314-321, 1907.

Sathyanarayana, S. et al., Electrodeposition From Sulpharnate Solutions,part III, Nickel-Zinc Alloys, Bull. India sect. Electrochem. Soc., vol.5, No. 4, 83-85 (1956).

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examin r.

US. Cl. X.R.

